Circuit and method for detecting ac voltage pulses

ABSTRACT

A detection circuit and method for detecting AC voltage pulses at a defined frequency relate to first transforming an input signal to a low-frequency signal by multiplying the input signal by a mixing frequency. This down-mixed signal can then be filtered and evaluated. The circuit is particularly suitable for identifying charge unit signals in the telephone network.

PRIORITY INFORMATION

This application is a Continuation of U.S. Application Ser. No.10/431,901, filed May 8, 2003, now published as U.S. Patent ApplicationPublication No. 2003/0207677 A1 on Nov. 6, 2003, which application andpublication are incorporated herein by reference.

BACKGROUND Field of the Invention

The invention relates to a circuit for detecting AC voltage pulses at adefined frequency, to a telephone having such a circuit, and to a methodfor identifying AC voltage pulses at a defined frequency.

In the European analog telephone network, charge unit pulses aretransmitted to the subscriber terminals, in order to allow theaccumulating charges to be detected at the subscriber end. The chargeunit pulses are short AC voltage pulses at a frequency of 16 kHz, or insome countries at about 12 kHz. These charge pulses must be identifiedand counted in the subscriber device.

Originally, the charge unit signals were output from the reception pathby a bandpass filter, and were supplied to a mechanical meter, which wasincremented by one step for each pulse. More recent solutions providefor the charge unit signals to be output from the reception path whichis provided for audio signal processing, and to be filtered by abandpass filter. The charge unit signals are then converted by acomparator to square-wave signals, whose period duration is determineddigitally by a counter. If the digitally detected length of the chargepulse is within a predefined tolerance band, a charge pulse isregistered.

This solution has the disadvantage of requiring a relatively largenumber of discrete components for outputting and bandpass filtering thecharge unit signals. Discrete components are expensive and impedefurther miniaturization of the subscriber terminals.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a circuit fordetecting AC voltage pulses at a defined frequency, a telephone havingsuch a circuit, and a method for identifying AC voltage pulses at adefined frequency, which overcome the above-mentioned disadvantages ofthe prior art apparatus and methods of this general type.

In particular, it is an object of the invention to further reduce thenumber of discrete components required to detect AC voltage pulses at adefined frequency.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a circuit for detecting AC voltage pulsesat a defined frequency. The circuit includes: an analog/digitalconverter unit for converting an input signal to a sequence of samplevalues; at least one mixing stage for multiplying the sequence of samplevalues by a mixing frequency signal; at least one filter stage fordefining a permissible intermediate frequency band and for obtainingconverted sample values by filtering the sequence of sample valueshaving been multiplied by the mixing signal; and an evaluation unit fordetermining whether an AC voltage pulse is present by using theconverted sample values.

The detection circuit has an analog/digital converter unit forconverting the input signal to a sequence of sample values. The sequenceof sample values is multiplied by a mixing frequency signal using atleast one mixing stage. Furthermore, the detection circuit has at leastone filter stage for defining the permissible intermediate frequencyband. The converted sample values are used by an evaluation unit todetermine whether an AC voltage pulse is present.

In contrast to the situation in the prior art, the incoming AC voltagepulse is first of all digitized; the rest of the processing is thencarried out in digital form. Both the mixing stage or the mixing stagesand the filter stage or the filter stages are in the form of digitalcircuits or a digital signal processor.

The superheterodyne principle is used in order to allowfrequency-specific detection of AC voltage pulses. This is done bymultiplying the sample values of the input signal by a mixing frequencysignal at the frequency ω_(M). An AC voltage pulse at the frequency ω₀is thus on the one hand up-mixed to the frequency ω₀+ω_(M), and on theother hand is down-mixed to the frequency ω₀−ω_(M). A downstream filterstage defines the permissible intermediate frequency band and thus thetolerance band for the frequency of the AC voltage pulse. For example,the up-mixed component ω₀+ω_(M) can be suppressed by a low-pass filter.

A low-pass filter can also be used to define the permissibleintermediate frequency band for the down-mixed component ω₀−ω_(M),because |ω₀−ω_(M)|≦ω_(G) and thus ω_(M)−ω_(G)≦ω₀≦ω_(M)+ω_(G) at acut-off frequency ω_(G). Instead of a low-pass filter, a filter with adifferent characteristic, for example, a bandpass filter, may also beused to define the permissible intermediate frequency band.

The AC voltage signal to be detected is thus first of all down-mixed tothe intermediate frequency band, and the frequency band within which ACvoltage pulses are intended to be detected is then defined by limitingthe intermediate frequency band. The evaluation unit then uses theconverted sample values to determine whether an AC voltage pulse hasoccurred in the frequency window defined in this way.

The use of the superheterodyne principle for detecting AC voltage pulsesat a defined frequency means that there is no need for filterarrangements with discrete components. The signal processing is carriedout completely digitally and can be carried out by digital signalprocessors, thus allowing the evaluation circuit to be implemented in aspace-saving and cost-effective manner.

According to one advantageous embodiment of the invention, the circuitfor detecting AC voltage pulses has at least one decimation stage, whichreduces the signal rate by omitting sample values. Decimation stagessuch as these allow the sampling rate for the sample values to bereduced in steps; the actual signal processing can then be carried outat a lower sampling frequency. More processing time is thus availablefor signal processing for each sample value, and the requirements forthe performance of the digital signal processor or of the digitalevaluation circuit can therefore be made less stringent. A furtheradvantage of a low processing frequency is that the power consumption ofthe DSP or of the evaluation circuit is reduced.

A further advantageous embodiment of the invention provides for themixing stage or the mixing stages each to have a quadrature signal pathand an in-phase signal path. The sequence of sample values is multipliedby the sine signal at the mixing frequency in the quadrature signalpath, and the sequence of sample values is multiplied by the cosinesignal at the mixing frequency in the in-phase signal path.

The cosine signal of the mixing frequency, which is used in the in-phasesignal path, is phase-shifted through 90° relative to the sine signalwhich is used in the quadrature signal path. An AC voltage signal whichis applied to the input of the evaluation circuit can thus be processedirrespective of its phase angle relative to the mixing frequency signal.Either the quadrature signal path or the in-phase signal path producesthe stronger signal, depending on the phase angle of the input signal.

In this case, it is particularly advantageous for the multiplication bythe sine signal at the mixing frequency to be carried out by multiplyingthe sequence of sample values by the sequence 0, 1, 0, −1, 0. . . . Thisvalue sequence corresponds to the profile of the sine signal at themixing frequency in the situation where four times the mixing frequencyis chosen as the sampling frequency. If the mixing frequency is 16 kHz,the sampling frequency must therefore be chosen to be 64 kHz. Themultiplication of the sequence of sample values by the sequence 0, 1, 0,−1, 0 . . . can be carried out in a simple manner by every alternatesample value being set to zero and every fourth sample value beinginverted. The multiplication of the sequence of sample values by thesequence 0, 1, 0, −1, 0 . . . can be linked in a simple manner to adecimation step, by first of all omitting every alternate sample value.Every alternate sample value of the remaining sample values is theninverted.

In a corresponding manner the multiplication by the cosine signal at themixing frequency can be carried out by multiplying the sequence ofsample values by the sequence 1, 0, −1, 0, 1, . . . , if the samplingfrequency is four times as great as the mixing frequency. This takesaccount of the 90° phase shift between the sine signal and the cosinesignal.

It is advantageous for the evaluation unit to in each case square, addand compare with a threshold value, the converted sample values in thequadrature signal path and in the in-phase signal path. Once the ACvoltage pulse has passed through the various mixing and filter stages,the square of the magnitude of the signal can be obtained by squaringand adding the converted sample values from the quadrature signal pathand from the in-phase signal path. The splitting of the signal path intoa quadrature signal path and an in-phase signal path always results inthe same square of the magnitude irrespective of the phase angle of theinput signal relative to the mixing signal.

In the situation where the AC voltage pulse is within the permissibletolerance band and passes through the various mixing and filter stages,the threshold value which is defined in the evaluation unit is exceeded,which means that an AC voltage pulse of a specific type has beenreceived. If, on the other hand, the frequency of the AC voltage pulseis not within the predefined frequency window, then the signal is soseverely attenuated by the mixing and filter stages that the square ofthe magnitude of the signal is below the threshold value.

It is advantageous for the mixing stage to multiply the sequence ofsample values by a mixing frequency signal whose frequency correspondsto the frequency of the AC voltage pulses to be detected. The mixingfrequency signal ω_(M) results in an AC voltage pulse at the frequencyω₀ being down-mixed to the frequency ω₀−ω_(M). If the frequency of themixing signal corresponds to the frequency of the AC voltage pulses tobe detected, then the AC voltage signal which is present initially isdown-mixed to 0 Hz, or to a very low frequency.

This down-mixed signal can be processed more easily than the originalrelatively high-frequency signal. In accordance with the Nyquisttheorem, the down-mixed signal can be detected using relatively lowsampling rates. The down-mixing of the input signal thus allows thesampling frequency to be reduced. A further advantage is that thedown-mixed, low-frequency signal can be band-limited by using a low-passfilter stage, in order in this way to define the permissible frequencywindow for the AC voltage signal which was present initially. Thisfrequency window extends from ω_(M)−ω_(G) to ω_(M)+ω_(G) where ω_(M) isthe mixing frequency and ω_(G) is the cut-off frequency of the low-passfilter.

According to a further advantageous embodiment of the invention, thecircuit has a first mixing stage which multiplies the sequence of samplevalues by a first mixing frequency signal having a frequencycorresponding to the frequency of a first type of AC voltage pulses.Furthermore, the circuit has a second mixing stage, which is arrangedbetween the first mixing stage and the evaluation unit, and whichmultiplies the sequence of sample values by a second mixing frequencysignal. The frequency of the second mixing frequency signal correspondsto the difference between the frequencies of the first type and of asecond type of AC voltage pulses.

An AC voltage pulse of the first type which is applied to the input isdown-mixed by the first mixing stage to a frequency of 0 Hz, or to avery low frequency, and can then be processed further by a firstdetection path. If, however, an AC voltage pulse of the second type isapplied to the input of the first mixing stage, then this AC voltagepulse is down-mixed to a frequency which corresponds to the differencefrequency between the first and the second type of AC voltage pulses.This signal is supplied to the downstream second mixing stage, whosemixing frequency corresponds precisely to this difference frequency. Thesecond mixing stage then down-mixes the applied difference frequencysignal to 0 Hz or to a very low frequency. The signal which is producedat the output of the second mixing stage is processed further by thesecond detection path. Filter stages can be arranged both in the firstdetection path (for the first type of AC voltage pulses) and in thesecond detection path (for the second type of AC voltage pulses) inorder in each case to define the permissible frequency bands for thefirst type and for the second type of AC voltage pulses.

It is particularly advantageous for a low-pass filter stage whosecut-off frequency is above the difference frequency to be arrangedbetween the first mixing stage and the second mixing stage. Not only thedown-mixed signal ω₀−ω_(M), but also an up-mixed signal at the frequencyω₀+ω_(M) are produced at the output of the second mixing stage. Thelow-pass filter stage, which is arranged between the first mixing stageand the second mixing stage, suppresses this up-mixed signal component,since it is not required for the further signal processing.

It is particularly advantageous for the circuit to be used for detectingcharge unit signals in the telephone network. The charge unit signalsare short AC voltage pulses at a defined frequency of about 16 kHz, orin some countries at about 12 kHz. If the detection circuit is designedexclusively for processing 16 kHz pulses, then a single mixing stagewith a mixing frequency of 16 kHz is sufficient to down-mix the inputsignal to low frequencies. In a corresponding way, a mixing stage whichoperates at a mixing frequency of 12 kHz is sufficient for detecting 12kHz pulses. If, on the other hand, a single detection circuit isintended to be used to detect both 12 kHz and 16 kHz pulses, then theinput signal can first of all be down-mixed by a first mixing stage,which is operated at a mixing frequency of 16 kHz, to 0 Hz (when thesignal is at 16 kHz), or to 4 kHz (when the signal is as 12 kHz). The 16kHz signal which has been down-mixed to 0 Hz can be evaluatedimmediately. The 12 kHz signal which has been down-mixed to 4 kHz canlikewise be down-mixed to 0 Hz by a second mixing stage, which isoperated at a mixing frequency of 4 kHz, and can then be evaluated.

The detection circuit for AC voltage pulses at a defined frequency isparticularly suitable for being used in a telephone, in order to detectthe charge unit signals that are transmitted via the telephone network.A telephone that is equipped with the detection circuit can be matchedto all the charge unit signal standards that are used in Europe. Theproduction costs are in this case lower than for previous solutions.

In this case, it is particularly advantageous for the charge unitsignals to be detected via the speech path, via which the audio signalprocessing is also carried out. In contrast to the situation withprevious solutions, there is no need to output the charge unit pulsesfrom the speech path, and filter arrangements with discrete componentsare superfluous for the inventive solution.

In particular, it is advantageous for the charge unit signals to bedigitized by the analog/digital converter unit in the speech path. Modemtelephones have an analog/digital converter unit, which converts theincoming audio signal to digital sample values. In the inventivesolution, this analog/digital converter unit also carries out the taskof digitizing the incoming charge unit signals. No separateanalog/digital converter module is therefore required for theanalog/digital conversion of the charge unit signals.

It is particularly advantageous for the various processing stages forconverting the sequence of sample values to be in the form of one ormore digital signal processors. The mixing stages, filter stages anddecimation stages which are required for evaluating the charge unitsignals may be formed completely by digital signal processors. A digitalsignal processor can also be used for the evaluation unit in which thesample values are squared and added.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a telephone portion including a circuitfor detecting charge unit signals at a defined frequency. The circuitincludes: an analog/digital converter unit for converting an inputsignal to a sequence of sample values; at least one mixing stage formultiplying the sequence of sample values by a mixing frequency signal;at least one filter stage for defining a permissible intermediatefrequency band and for obtaining converted sample values by filteringthe sequence of sample values having been multiplied by the mixingsignal; and an evaluation unit for determining whether a charge unitsignal is present by using the converted sample values.

In accordance with an added feature of the invention, a speech path isprovided via which audio signal processing is performed; and the chargeunit signals are detected via the speech path.

In accordance with an additional feature of the invention, theanalog/digital converter unit is configured in the speech path; and thecharge unit signals are digitized by the analog/digital converter unit.

In accordance with another feature of the invention, at least onedigital signal processor is provided for implementing at least themixing stage and the filter stage.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for identifying different typesof AC voltage pulses each having a defined frequency. The methodincludes steps of: digitizing an input signal to form a sequence ofdigitized sample values; obtaining a sequence of converted sample valuesby converting the sequence of sample values using any desired sequenceof steps including multiplying the sequence of sample values bypredetermined mixing frequency signals and filtering the sequence ofsample values to define a permissible intermediate frequency band; andusing the sequence of converted sample values to determine whether aspecific type of AC voltage pulse is present.

In accordance with an added feature of the invention, the methodincludes, while performing the step of converting the sequence of samplevalues, reducing a signal rate by reducing a number of the sample valuesin the sequence of sample values.

In accordance with an additional feature of the invention, the step ofdetermining whether a specific type of AC voltage pulse is presentincludes squaring, adding and comparing the sequence of converted samplevalues with a threshold value.

In accordance with another feature of the invention, the step ofmultiplying the sequence of sample values by predetermined mixingfrequency signals includes multiplying the sequence of sample values bya mixing frequency signal having a frequency corresponding to afrequency of the AC voltage pulses being identified.

In accordance with a further feature of the invention, the step ofmultiplying the sequence of sample values by predetermined mixingfrequency signals includes first multiplying the sequence of samplevalues by a first mixing frequency signal having a frequencycorresponding to a frequency of a first type of AC voltage pulses, andsubsequently multiplying the sequence of sample values by a secondmixing frequency signal having a frequency corresponding to a differencebetween the frequency of the first type of AC voltage pulses and afrequency of a second type of AC voltage pulses.

In accordance with a further added feature of the invention, the ACvoltage pulses are charge unit signals in a telephone network.

In the method for identifying different types of AC voltage pulses, eachat a defined frequency, the input signal is digitized in a first step.The sequence of digitized sample values obtained in this way is thenconverted by carrying out various steps including the multiplication ofthe sequence of sample values by predetermined mixing frequency signalsand filtering the sequence of sample values. The permissibleintermediate frequency band is defined by filtering the sequence ofsample values. It is then possible to use the sequence of convertedsample values to determine whether a specific type of AC voltage pulseis present.

The inventive method allows completely digital selective identificationof various types of AC voltage pulses. In the method, the input signalis down-mixed to low frequencies by being multiplied by different mixingfrequency signals. First, this has the advantage that the signal can befiltered considerably more easily in the low-frequency intermediatefrequency band than in the original input signal frequency band. Afurther advantage is that it is possible to use considerably lowersignal rates once the input signal has been transformed to thelow-frequency band. This means that less powerful signal processors canbe used and that the power consumption of the circuit is reduced.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin an evaluation circuit for AC voltage pulses, it is nevertheless notintended to be limited to the details shown, since various modificationsand structural changes may be made therein without departing from thespirit of the invention and within the scope and range of equivalents ofthe claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The sole drawing FIG. 1 shows a detection circuit that can detect both16 kHz and 12 kHz charge unit signals.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the sole drawing FIG. 1, there is shown a detectioncircuit for detecting AC voltage pulses at a defined frequency. Thisdetection circuit is particularly suitable for detecting the charge unitsignals that are transmitted via the telephone network. The circuit andthe detection principle are not, however, restricted to thisapplication, but can be used wherever identification pulses at a definedfrequency should be selectively detected.

The analog input signal 1 is first of all supplied to the analog/digitalconverter A/D and is converted to a sequence of digital sample valuesx(k). If the circuit in a telephone is used for detecting the chargeunit pulses, then the charge unit signals are digitized by the sameanalog/digital converter that is also used to convert the speech signalto digital values, because the charge unit pulses are not output fromthe speech path in this inventive solution.

The digitized sample values x(k) are now supplied to the first mixingstage, and for this purpose are split into a quadrature signal path 2and an in-phase signal path 3. In the mixer 4 for the quadrature signalpath, the signal is multiplied by the sine signal sin(ω_(M1)·t) at themixing frequency ω_(M1), where $\frac{1}{2\quad\pi} \cdot 16$ω_(M1) is chosen to be 16 kHz, thus corresponding to the frequency ofthe 16 kHz charge unit pulses.

In the mixer 5 for the in-phase signal path, the signal is multiplied bythe cosine signal cos(ω_(M1)·t) at the mixing frequency$\omega_{M\quad 1} = {\frac{1}{2\quad\pi} \cdot 16}$kHz, that is to say by a mixing signal which has been phase-shiftedthrough 90°. The charge unit pulses can therefore be detectedindependently of their phase angle relative to the mixing frequencysignals.

The signal rate at the output of the analog/digital converter and in themixers 4 and 5 is 64 kHz and is therefore four times the mixingfrequency f_(M1). Sampling is therefore carried out four times in eachclock cycle of the mixing frequency, therefore resulting in the sequence0, 1, 0, −1, 0, . . .$\left( {{\sin\quad\left( \frac{\pi \cdot k}{2} \right)},{{{where}\quad k} = 0},1,2,\ldots} \right)$as the sine signal at the mixing frequency. In a corresponding manner,the sequence 1, 0, −1, 0, 1, . . .$\left( {{\cos\quad\left( \frac{\pi \cdot k}{2} \right)},{{{where}\quad k} = 0},1,2,\ldots} \right)$is obtained for the cosine signal at the mixing frequency. Themultiplication of the input signal by the sine sequence in the mixer 4thus results in a signal sequence in which every alternate value is setto be equal to zero. A signal sequence in which every alternate value isequal to zero is also produced at the output of the mixer 5 of thein-phase signal path. In the subsequent decimation stages 6 and 7, thesignal rate is in each case reduced from f_(S)=64 kHz to f_(S)=32 kHz,by omitting those values which have been set to zero in the signalsequence.

An up-mixed signal in each case appears at the outputs of the mixers 4and 5, whose frequency corresponds to the sum of the frequencies of theinput signal and of the mixing signal, as well as a down-mixed signalwhose frequency corresponds to the difference frequency between theinput signal and the mixing signal. A 16 kHz pulse is on the one handup-mixed to 32 kHz by the mixing frequency f_(M1)=16 kHz, and is on theother hand down-mixed to 0 kHz (superheterodyne principle). In acorresponding manner, a 12 kHz pulse which is applied to the input ofthe first mixing stage is on the one hand up-mixed to 28 kHz, and is onthe other hand down-mixed to 4 kHz, by the mixing frequency of 16 kHz.The-low-pass filters 8 and 9 (TP1) have the task of filtering out theup-mixed frequency components and of allowing only the 0 kHz componentof the 16 kHz pulse as well as the 4 kHz component of the 12 kHz pulseto pass. It is therefore worthwhile to define the cut-off frequency ofthe low-pass filters 8 and 9 (TP1) in the quadrature and in-phase path,respectively, to be 4.5 kHz. The low-pass filters 8 and 9 are followedby the decimation stages 10 and 11, which once again reduce the signalfrom f_(S)=32 kHz to f_(S)=16 kHz. The repeated reduction in the signalrate allows the further signal processing to be significantlysimplified.

The following text is based on the assumption that the detection circuitfor charge unit pulses has been set to identify 16 kHz pulses. In thiscase, there is no need to use a second mixing frequency to once againup-mix or down-mix the signals which are produced at the output of thedecimation stages 10 and 11, because the 16 kHz pulses have already beendown-mixed by the mixers 4 and 5 from 16 kHz to 0 kHz.

The mixer 12 is thus switched to be inactive, or multiplies every valueapplied to it by 1. The actual band limiting of the down-mixed signal iscarried out by the low-pass filters 13 and 14 (TP2/TP3, TP2). Thelow-pass filters 13, 14 are used to define the intermediate frequencyband within which AC voltage pulses are intended to be detected. If, forexample, the cut-off frequency of the low-pass filters 13 and 14 isfixed at 0.5 kHz, then AC voltages at frequencies from −0.5 kHz to +0.5kHz can pass through the low-pass filters. This corresponds to apermissible frequency band from 15.5 kHz to 16.5 kHz in the inputsignal. The calculation example shows that the permissible band rangefor AC voltage pulses to be detected can be defined in a simple mannerby low-pass filtering the intermediate frequencies.

The sampling frequency is once again halved in the subsequent decimationstages 15 and 16 (from f_(S)=16 kHz to f_(S)=8 kHz), by omitting everyalternate signal value. The signal values which then remain are suppliedto the multipliers 17 and 18, which square each signal value. The adder19 in each case forms the sum of the squares of the quadrature andin-phase signal values, and thus determines the square of the magnitudeof the respective signal value, to be precise independently of the phaseangle of the input signal. The threshold value decision device 20compares the result supplied from the adder 19 with a predeterminedthreshold value. If this threshold value is exceeded, then the detectionsignal y₁₆(k) from the threshold value decision device 20 indicates thata 16 kHz pulse has been received.

Charge unit pulses at a frequency of about 12 kHz are used in someEuropean countries. If the circuit illustrated in the drawing FIG. 1 isintended to evaluate such 12 kHz pulses, then, in addition to the firstmixing frequency of f_(M1)=16 kHz, a second mixing frequency of f_(M2)=4kHz must also be used. For this purpose, the quadrature signal which isproduced at the output of the decimation stage 10 and has beendown-mixed by the mixer 4 to an intermediate frequency of 4 kHz issupplied to the mixers 12 and 21. The mixers 12 and 21 multiply thesignal applied to them by the sine signal sequence and cosine signalsequence, respectively, for the mixing frequency f_(M2)=4 kHz.

Since the sampling frequency f_(S) is in this case 16 kHz and is thusfour times the mixing frequency, the cosine signal sequence once againbecomes1. 0, −1, 0, 1, . . .$\left( {{\cos\quad\left( \frac{\pi \cdot k}{2} \right)},{{{where}\quad k} = 0},1,2,\ldots} \right),$and the sine sequence becomes0. 1, 0, −1, 0, . . .$\left( {{\sin\quad\left( \frac{\pi \cdot k}{2} \right)},{{{where}\quad k} = 0},1,2,\ldots} \right).$

Multiplication by a mixing frequency signal at the frequency 4 kHzresults in the intermediate frequency signal which is applied to themixers 12 and 21 on the one hand being up-mixed to 8 kHz and on theother hand being down-mixed to 0 kHz. The low-pass filters 13 and 22(TP2/TP3, TP3) which are arranged downstream in the signal path on theone hand eliminate the up-mixed 8 kHz component. On the other hand, withregard to the signal component which has been down-mixed to 0 kHz, thecut-off frequency of the low-pass filters 13 and 22 defines thepermissible intermediate band range within which pulses are detected.

The downstream decimation stages 15 and 23 halve the signal rate fromf_(S)=16 kHz to f_(S)=8 kHz by omitting every alternate sample value.The evaluation is carried out by the multipliers 17 and 24, by the adder25 and by the threshold value decision device 26. If the sum of thesquares of the signal values is greater than a predetermined threshold,then a 12 kHz pulse is present. This is indicated by the detectionsignal y₁₂(k) being produced at the output of the threshold valuedecision device 26.

1. A circuit for detecting AC voltage pulses at a defined frequency,comprising: an analog/digital converter unit for converting an inputsignal to a sequence of sample values; at least one mixing stage formultiplying the sequence of sample values by a mixing frequency signal,said mixing stage having a quadrature signal path and an in-phase signalpath; at least one filter stage for defining a permissible intermediatefrequency band and for obtaining converted sample values by filteringthe sequence of sample values having been multiplied by the mixingfrequency signal; and an evaluation unit for determining whether an ACvoltage pulse is present by using the converted sample values, saidevaluation unit squaring, adding and comparing the converted samplevalues in said quadrature signal path and in said in-phase signal pathwith a threshold value.
 2. The circuit according to claim 1, furthercomprising at least one decimation stage for reducing a signal rate byomitting sample values.
 3. The circuit according to claim 2, wherein: insaid quadrature signal path the sequence of sample values is multipliedby a sine signal at the mixing frequency signal; and in said in-phasesignal path the sequence of sample values is multiplied by a cosinesignal at the mixing frequency signal.
 4. The circuit according to claim3, wherein in said quadrature signal path, the sequence of sample valuesis multiplied by the sine signal by multiplying the sequence of samplevalues by a sequence 0, 1, 0, −1, 0, . . . .
 5. The circuit according toclaim 3, wherein in said in-phase signal path, the sequence of samplevalues is multiplied by the cosine signal by multiplying the sequence ofsample values by a sequence 1, 0, −1, 0, 1, . . . .
 6. The circuitaccording to claim 1, wherein the mixing frequency signal of said mixingstage has a frequency corresponding to the frequency of the AC voltagepulses that will be detected.
 7. The circuit according to claim 1,further comprising: a first mixing stage defining said mixing stage,said first mixing stage for multiplying the sequence of sample values bya first mixing frequency signal having a frequency corresponding to afrequency of a first type of AC voltage pulses; and a second mixingstage configured between said first mixing stage and said evaluationunit; said second mixing stage for multiplying the sequence of samplevalues by a second mixing frequency signal having a frequencycorresponding to a difference frequency formed by a difference betweenthe frequency of the first type of AC voltage pulses and a frequency ofa second type of AC voltage pulses.
 8. The circuit according to claim 7,further comprising: a low-pass filter stage having a cut-off frequencyabove the difference frequency; said low-pass filter stage configuredbetween said first mixing stage and said second mixing stage.
 9. Thecircuit according to claim 1, wherein the AC voltage pulse is a chargeunit signal in a telephone network.
 10. A telephone portion, comprising:an analog/digital converter unit for converting an input signal to asequence of sample values; at least one mixing stage coupled to receivethe sequence of sample values and to multiply the sequence of samplevalues by a mixing frequency signal; at least one filter stage coupledto filter the sequence of sample values multiplied by the mixingfrequency signal to define a permissible intermediate frequency band andto provide converted sample values; an evaluation unit coupled toreceive the converted sample values to determine whether a charge unitsignal is present; and a speech path coupled to for performing audiosignal processing, the charge unit signal being detected via said speechpath.
 11. The telephone portion according to claim 10, wherein: saidanalog/digital converter unit is configured in the speech path; and thecharge unit signals are digitized by said analog/digital converter unit.12. The telephone portion according to claim 10, further comprising atleast one digital signal processor for implementing at least said mixingstage and said filter stage.
 13. A method for identifying differenttypes of AC voltage pulses each having a defined frequency, the methodwhich comprises: digitizing an input signal to form a sequence ofdigitized sample values; obtaining a sequence of converted sample valuesby converting the sequence of sample values using any desired sequenceof steps including multiplying the sequence of sample values bypredetermined mixing frequency signals and filtering the sequence ofsample values to define a permissible intermediate frequency band; andusing the sequence of converted sample values to determine whether aspecific type of AC voltage pulse is present by squaring, adding andcomparing the sequence of converted sample values with a thresholdvalue.
 14. The method according to claim 13, which further comprises,while performing the step of converting the sequence of sample values,reducing a signal rate by reducing a number of the sample values in thesequence of sample values.
 15. The method according to claim 13, whereinthe step of multiplying the sequence of sample values by predeterminedmixing frequency signals includes multiplying the sequence of samplevalues by a mixing frequency signal having a frequency corresponding toa frequency of the AC voltage pulses being identified.
 16. The methodaccording to claim 13, wherein the step of multiplying the sequence ofsample values by predetermined mixing frequency signals includes firstmultiplying the sequence of sample values by a first mixing frequencysignal having a frequency corresponding to a frequency of a first typeof AC voltage pulses, and subsequently multiplying the sequence ofsample values by a second mixing frequency signal having a frequencycorresponding to a difference between the frequency of the first type ofAC voltage pulses and a frequency of a second type of AC voltage pulses.17. The method according to claim 13, wherein the AC voltage pulses arecharge unit signals in a telephone network.
 18. A detection circuit,comprising: an analog/digital converter to convert an input signal to asequence of sampled values; a mixing stage having a quadrature signalpath and an in-phase signal path and multiplying said sequence ofsampled values by a mixing frequency signal to provide of mixed sampledvalues; a filter stage coupled to receive said mixed sampled values andto filter said mixed sampled values to converted sample values; and anevaluation unit coupled to receive mixed sampled values, the evaluationunit including a circuit to square, and add said converted sample valuesand compare results with a threshold value to determine whether an ACvoltage pulse is present.
 19. A telephone, comprising: an analog/digitalconverter to convert an input signal in a speech path of the telephoneto a sequence of sampled values; a mixing stage to multiply saidsequence of sampled values by a mixing frequency signal to provide mixedsampled values at an output of the mixing stage, the mixing frequencyhaving a value determined by a frequency of a charge unit signal for thetelephone; a filter stage coupled to receive said mixed sampled valuesand filter said mixed sampled values to converted sampled values; anevaluation unit coupled to receive said converted sample values anddetermine whether the charge unit signal is present.
 20. A method ofprocessing an input signal to detect an AC pulse having a predeterminedfrequency, comprising: digitizing the input signal to a sequence ofsample values; multiplying said sequence of sample values by a mixingfrequency signal having a frequency determined by the frequency of theAC pulse to provide mixed sample values, using a mixing stage having aquadrature signal path and an in phase signal path; filtering said mixedsample values to provide converted sample values; and squaring, addingand comparing said converted sample values with a threshold value, anddetermining whether the AC voltage pulse is present.
 21. A circuit fordetecting AC voltage pulses at a defined frequency, comprising: ananalog/digital converter unit for converting an input signal to asequence of sample values; at least one mixing stage for multiplying thesequence of sample values by a mixing frequency signal, said mixingstage has a quadrature signal path in which the sequence of samplevalues is multiplied by a sine signal at the mixing frequency; and saidmixing stage has an in-phase signal path in which the sequence of samplevalues is multiplied by a cosine signal at the mixing frequency; atleast one filter stage for defining a permissible intermediate frequencyband and for obtaining converted sample values by filtering the sequenceof sample values having been multiplied by the mixing frequency signal;an evaluation unit for determining whether an AC voltage pulse ispresent by using the converted sample values wherein said evaluationunit squares, adds and compares converted sample values in saidquadrature signal path and in said in-phase signal path with a thresholdvalue; and at least one decimation stage for reducing a signal rate byomitting sample values.
 22. The circuit according to claim 21, whereinin said quadrature signal path, the sequence of sample values ismultiplied by the sine signal by multiplying the sequence of samplevalues by a sequence 0, 1, 0, −1, 0, . . . .
 23. The circuit accordingto claim 21 , wherein in said in-phase signal path, the sequence ofsample values is multiplied by the cosine signal by multiplying thesequence of sample values by a sequence 1, 0, −1, 0, 1, . . . .
 24. Thecircuit according to claim 21, wherein the mixing frequency signal ofsaid mixing stage has a frequency corresponding to the frequency of theAC voltage pulses that will be detected.
 25. The circuit according toclaim 21, further comprising: a first mixing stage defining said mixingstage, said first mixing stage for multiplying the sequence of samplevalues by a first mixing frequency signal having a frequencycorresponding to a frequency of a first type of AC voltage pulses; and asecond mixing stage configured between said first mixing stage and saidevaluation unit; said second mixing stage for multiplying the sequenceof sample values by a second mixing frequency signal having a frequencycorresponding to a difference frequency formed by a difference betweenthe frequency of the first type of AC voltage pulses and a frequency ofa second type of AC voltage pulses.
 26. The circuit according to claim25, further comprising: a low-pass filter stage having a cut-offfrequency above the difference frequency; said low-pass filter stageconfigured between said first mixing stage and said second mixing stage.27. The circuit according to claim 21, wherein the AC voltage pulse is acharge unit signal in a telephone network.
 28. A telephone portionincluding a circuit for detecting charge unit signals at a definedfrequency, the circuit comprising: an analog/digital converter unit forconverting an input signal to a sequence of sample values; at least onemixing stage for multiplying the sequence of sample values by a mixingfrequency signal; at least one filter stage for defining a permissibleintermediate frequency band and for obtaining converted sample values byfiltering the sequence of sample values having been multiplied by themixing signal; an evaluation unit for determining whether a charge unitsignal is present by using the converted sample values; and a speechpath via which audio signal processing is performed; the charge unitsignals being detected via the speech path.
 29. The telephone portionaccording to claim 28, wherein: said analog/digital converter unit isconfigured in the speech path; and the charge unit signals are digitizedby said analog/digital converter unit.
 30. The telephone portionaccording to claim 29, further comprising: at least one digital signalprocessor for implementing at least said mixing stage and said filterstage.
 31. A method for identifying different types of AC voltage pulseseach having a defined frequency, the method which comprises: digitizingan input signal to form a sequence of digitized sample values; obtaininga sequence of converted sample values by converting the sequence ofsample values using any desired sequence of steps including multiplyingthe sequence of sample values by predetermined mixing frequency signalsand filtering the sequence of sample values to define a permissibleintermediate frequency band and which further comprises, whileperforming the step of converting the sequence of sample values,reducing a signal rate by reducing a number of the sample values in thesequence of sample values; and using the sequence of converted samplevalues to determine whether a specific type of AC voltage pulse ispresent wherein the step of determining whether a specific type of ACvoltage pulse is present includes squaring, adding and comparing thesequence of converted sample values with a threshold value.
 32. Themethod according to claim 31, wherein the step of multiplying thesequence of sample values by predetermined mixing frequency signalsincludes multiplying the sequence of sample values by a mixing frequencysignal having a frequency corresponding to a frequency of the AC voltagepulses being identified.
 33. The method according to claim 31, whereinthe step of multiplying the sequence of sample values by predeterminedmixing frequency signals includes first multiplying the sequence ofsample values by a first mixing frequency signal having a frequencycorresponding to a frequency of a first type of AC voltage pulses, andsubsequently multiplying the sequence of sample values by a secondmixing frequency signal having a frequency corresponding to a differencebetween the frequency of the first type of AC voltage pulses and afrequency of a second type of AC voltage pulses.
 34. The methodaccording to claim 31, wherein the AC voltage pulses are charge unitsignals in a telephone network.